bus: uniphier-system-bus: revive tristate prompt

At first, commit 4b7f48d3 ("bus: uniphier-system-bus: add UniPhier
System Bus driver") introduced this driver as a tristate one.

Then, commit 326ea45a ("bus: uniphier: allow only built-in
driver") temporarily made it boolean in order to fix a link error
in case it is compiled as a module.

The root cause was fixed by commit b80443c2 ("of/platform: export
of_default_bus_match_table").

Now this driver can really be a module.
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Olof Johansson <olof@lixom.net>

drivers/bus/Kconfig

@@ -132,7 +132,7 @@ config SUNXI_RSB
 	  and AC100/AC200 ICs.
 
 config UNIPHIER_SYSTEM_BUS
-	bool "UniPhier System Bus driver"
+	tristate "UniPhier System Bus driver"
 	depends on ARCH_UNIPHIER && OF
 	default y
 	help